Glass diffraction grating and method of producing the same

This is a Continuation of International Patent Application No. PCT/JP2022/006859 filed Feb. 21, 2022, which designates the U.S., and which claims priority from U.S. Provisional Patent Application No. 63/165,361, dated Mar. 24, 2021. The contents of these applications are hereby incorporated by reference.

The present invention relates to a glass diffraction grating and a method of producing the same.

There is a need for a transmission diffraction optical element with a great angular dispersion used in astronomic observation in an artificial satellite orbit or in a spectroscope for a planetary probe, fore example. In order to produce a highly efficient transmission diffraction optical element with a great angular dispersion, a volume binary or a trapezoid diffraction grating with deep grooves made of glass that is highly radiation-resistant and that has an aspect ratio of 2 or greater and a period of 0.2 to 10 micrometers is required.

In the prior art, a method of producing a diffraction grating having deep grooves on a surface of quartz glass through plasma etching using a metal film of chromium (Cr) or the like as a mask is known. The method, however, has the following problems.

Firstly, since etching onto quartz glass is carried out through ion bombardment, the quartz glass is damaged in a process of producing a diffraction grating having deep grooves. Accordingly, it is difficult to flatten a surface of the grating to the level required in optics. In addition, particles of quartz glass removed through the plasma etching adhere to a wall surface and the like of the grating and therefore surface roughness is further deteriorated.

Secondly, the deeper the grooves, the higher resistance of a mask is required. In the case of a mask having a fine pattern, the groove contour tends to be tapered when the resistance of the mask is insufficient. When the thickness of a film of chrome (Cr) or the like is increased in order to increase the resistance of the mask, a crack and a separation are generated in the film itself.

Thirdly, when the grooves are deep, the number of ions that reach the bottom and ions that do not reach the bottom shave sides of ridges so that bowing appears in the sides of each ridge. Taper and bowing that have not been taken into account in the design deteriorate the optical performance.

Because of the above-described problems of the method, a diffraction grating with the aspect ratio of 2 or greater can hardly be produced by the above-described method.

As another method of producing a glass diffraction grating with a high aspect ratio, a method using a SOQ (Silicon on Quartz) substrate has been developed (Patent document 1, for example). In the method disclosed in Patent document 1, however, it is difficult to obtain silicon dioxide (quartz glass; SiO) by completely oxidizing silicon. A refractive index of portions obtained by incomplete oxidation (silicon monoxide: SiO, disilicon trioxide: SiO) is greater than that of quartz glass (SiO: nd=1.97, SiO: nd=1.46). Accordingly, characteristics are quite different from those of the design. Further, in a cooling process from a temperature in the process of oxidation (approximately 1000° C.) to the room temperature, the diffraction grating suffers warping because of a difference in coefficient of linear thermal expansion between silicon or silicon monoxide and silicon dioxide. In order to obtain a glass diffraction grating of sufficiently satisfactory quality in shape, material and characteristics, the manufacturing process has to be adjusted extremely precisely.

As sill another method, a method in which borosilicate glass is filled in a mold of silicon to produce a glass diffraction grating has been developed (Non-patent document 1, for example). However, the period of a grating produced by the above-described method is a few tens of micrometers and the period is approximately ten times as great as the period of a diffraction grating that is appropriate for the above-described purpose. Thus, by the above-described method a glass diffraction grating having a grating period of 10 micrometers or smaller that is appropriated for the above-described purpose cannot be produced.

Thus, a glass diffraction grating, the aspect ratio of grooves of the grating being 2 or greater and the period of the grating being 10 micrometers or smaller, and a method of producing the same have not been developed. Accordingly, there is a need for a glass diffraction grating, the aspect ratio of grooves of the grating being 2 or greater and the period of the grating being 10 micrometers or smaller, and a method of producing the same.

Non-patent document: A. Amnache and L. G. Frechette, “High-aspect ratio microstructures in borosilicate glass by molding and sacrificial silicon etching: capabilities and limits”, Solid-State Sensors, Actuators and Microsystems Workshop Hilton Head Island, South Carolina, Jun. 5-9, 2016

The object of the present invention is to provide a need for a glass diffraction grating, the aspect ratio of grooves of the grating being 2 or greater and the period of the grating being 10 micrometers smaller, and a method of producing the same.

A method of producing a diffraction grating according to a first aspect of the present invention is for producing a diffraction grating of borosilicate glass or barium borosilicate glass, the period of the grating being from 0.2 to 10 micrometers and the aspect ratio of grooves of the grating being 2 or greater. The method includes the steps of forming a grating on a surface of a silicon wafer through the Bosch process (cyclic etching); forming an oxide film on a surface of the grating by heating and exposure to water vapor of the silicon wafer; removing the oxide film using hydrofluoric acid; making the surface provided with the grating of the silicon wafer and a surface of a glass plate undergo anodic bonding in a container kept at the degree of vacuum of 0.01 to 0.1 pascals; heating the silicon wafer and the glass plate that have been bonded to each other so as to melt glass and to fill spaces formed between ridges of the grating of silicon with the molten glass; polishing a surface opposite to the boded surface of the silicon wafer and a surface opposite to the boded surface of the glass plate; and removing silicon from the glass plate by selective etching using xenon difluoride gas.

Since the producing method includes the step of forming an oxide film on a surface of the grating by heating and exposure to water vapor of the silicon wafer and the step of removing the oxide film using hydrofluoric acid after the Bosch process, scallops that have been generated in the Bosch process on a side of each ridge of the grating can be flattened and surface roughness of 10 nanometers or smaller can be realized. Accordingly, optical performance of the glass diffraction grating can be improved. Further, since the producing method includes the step of removing silicon from the glass plate by selective etching using xenon difluoride gas, the degree of purity of material of the glass diffraction grating can be improved. Accordingly, optical performance of the glass diffraction grating can be improved.

The method of producing a diffraction grating according to a first embodiment of the first aspect of the present invention further includes a thermal oxidation process in which the glass plate is made to undergo heating and exposure to water vapor after the step of removing silicon from the glass plate by selective etching.

According to the present embodiment, through the additional thermal oxidation process, oxides of silicon such as silicon monoxide (SiO) that has been left unetched can be changed to silicon dioxide (SiO) that has the same quality with glass.

In the method of producing a diffraction grating according to a second embodiment of the first aspect of the present invention, the step of heating the silicon wafer and the glass plate that have been bonded to each other is carried out using a hot isostatic pressing machine.

A glass diffraction grating according to a second aspect of the present invention is a diffraction grating of borosilicate glass or barium borosilicate glass, the period of the grating being from 0.2 to 10 micrometers and the aspect ratio of grooves of the grating being 2 or greater.

In the glass diffraction grating according to a first embodiment of the second aspect of the present invention, a radius of curvature of a side of a ridge of the diffraction grating, the ridge being in a substantially rectangular shape and the side being substantially in the direction in which the height of the diffraction grating is measured, is ten times as great as the period of the diffraction grating or greater.

By the shape of the diffraction grating of the present embodiment satisfactory performance can be obtained.

In the glass diffraction grating according to a second embodiment of the second aspect of the present invention, a ratio of the width of each ridge to the period of the grating rages from 0.1 to 0.9.

In the glass diffraction grating according to a third embodiment of the second aspect of the present invention, an arithmetic average roughness on a side of a ridge of the grating is 10 nanometers or smaller.

By the shape of the diffraction grating of the present embodiment satisfactory performance can be obtained.

In the glass diffraction grating according to a fourth embodiment of the second aspect of the present invention, in a cross section including a straight line in the direction in which the period of the diffraction grating is measured and a straight line in the direction in which the height of the diffraction grating is measured, an angle θ formed between a side in the direction in which the period is measured and a side substantially in the direction in which the height is measured of a ridge that is in a substantially rectangular shape is equal to or greater than 70 degrees and equal to or less than 88 degrees.

The reasons why the angle should preferably be equal to or greater than 70 degrees and equal to or less than 88 degrees are below.

Firstly, by changing the angle θ from the right angle to an acute angle, the efficiency of spectral diffraction of the p polarized wave in which the electric field oscillates in an incident plane containing the incident ray and the reflected ray and the efficiency of spectral diffraction of the s polarized wave in which the electric field oscillates in a plane perpendicular to the incident plane can be made closer to each other and consequently the total efficiency of diffraction can be improved.

Secondly, by changing the angle θ from the right angle to an acute angle, the filling of glass into the spaces formed by the grating of silicon between the silicon waferand the glass plateis more easily carried out in step Sas described later.

shows a glass diffraction grating according to an embodiment of the present invention. In a glass diffraction grating according to the embodiment, the grating period P ranges from 0.2 micrometers to 10 micrometers, the grating height h ranges from 0.4 micrometers to 200 micrometers and the aspect ratio of grooves h/w is 2 or greater. “w” represents a distance between ridges r of the grating. The duty ratio (P·w)/P rages from 0.1 to 0.9. Material of the grating is borosilicate glass or barium borosilicate glass.

is a flowchart for describing a method of producing a glass diffraction grating according to an embodiment of the present invention.

In step Sofa surface of a silicon waferis coated with photoresist and a grating pattern is drawn on the photoresist through mask lithography, laser beam lithography, electron beam lithography, a stepper, laser interferometric lithography, or the like. Then a grating is formed on the surface of the silicon waferby making the silicon undergo etching through the Bosch process.

shows a grating on the silicon waferformed after the Bosch process. On the top of each ridge of the grating photoresistremains.

is a SEM (scanning electron microscope) image of the silicon waferprovided with the grating formed after the Bosch process. The image ofcorresponds to. The scale division shown on the images ofand other drawings is 0.5 micrometers. The period of the grating is approximately 2 micrometers.

In step Softhe silicon waferprovided with the grating, from which the photoresist has been removed, is heated in a heating furnace to form an oxide film on the surface of the grating.

shows an electric furnaceused for heating the silicon wafer. The silicon waferis placed on a portin a quartz pipeand heated by a heaterprovided outside the quartz pipe. Inthe silicon waferis represented as W. By way of example, the heating temperature is 1000 degrees (° C.) and the heating time is 20 minutes. Oxygen and hydrogen are fed into the quartz pipethrough a gas inletand an oxide film is formed on the surface of the grating of silicon using water vapor generated by combustion. By way of example, a thickness of the oxide film is 350 nanometers.

shows a grating on the silicon waferafter the heating. An oxide filmhas been formed on the surface of the grating.

is a SEM image of the grating on the silicon waferafter the heating. The image ofcorresponds to.

In step Softhe oxide film on the surface of the grating is removed using hydrofluoric acid. More specifically, the oxide film is removed by immersing the silicon waferprovided with the grating into hydrofluoric acid in a container in a draft chamber.

shows a grating on the silicon waferafter the treatment with hydrofluoric acid.

Each ofandshows a SEM image of the grating on the silicon waferafter the treatment with hydrofluoric acid. The image ofis a side view of the grating and the image ofis a view from above. The images ofandcorrespond to.

The reason that an oxide film is formed on the surface of the grating in step Sand then the oxide film is removed in step Sis to reduce roughness on side surfaces of ridges of the grating. On side surfaces of ridges of the grating after the Bosch process shown in, plural scallops have been formed in the direction perpendicular to the direction of the height of the grating. The height of the scallops rages from several nanometers to several tens of nanometers. By having the oxide film to be formed and then to be removed, the plural scallops are removed so that the surface roughness is reduced. An arithmetic average roughness Ra after step Sis 10 nanometers or smaller. In step S, a ratio of a thickness of a portion of the oxide film formed outwardly from the position of the original surface of the silicon wafer and a thickness of a portion of the oxide film formed inwardly from the position is approximately 3 to 2. Since the oxide film is removed in step S, the above-described ratio should be taken in to account when the dimensions of the grating on the silicon wafer are determined in step Sand the thickness of the oxide film is determined in S.

In step Sof, the surface provided with the grating of the silicon waferand a surface of a glass plate are made to undergo anodic bonding in a vacuum.

Each ofshows a chamberin which the anodic bonding is carried out.

As shown in, the silicon waferprovided with the grating and a glass platethat sandwich spacersin a rod shape between the surface provided with the grating of the silicon waferand a surface of the glass plateare placed on a basein the chamber. The degree of vacuum in the chamberis made to be 0.01 to 0.1 pascals and the temperature is increased up to be 400° C. by heating. By the presence of the spacers a degree of vacuum in spaces between ridges of the grating is also made to be a value described above.

Then, as shown in Fi., the spacersare removed so as to bring the surface provided with the grating of the silicon waferand the surface of the glass plateinto contact with each other.

Then, as shown in Fi., a negative voltage of −500 to −1000 volts is applied across the glass plate via a pressure plateand the basewhile applying a pressure of approximately 10 kilopascals to the silicon waferand the glass platedby the pressure plate.

illustrates the principle of anodic bonding. Heating the glass plateenhances the mobility of sodium ions (Na) in borosilicate glass or barium borosilicate glass. When the silicon waferand the glass plateare brought into contact with each other and the silicon waferand the glass plateare connected respectively to the positive electrode and the negative electrode of a power source, the sodium ions move toward the negative electrode. As a result, in the glass platea sodium ion deficient layer is generated in the vicinity of the interface with the silicon wafer. Since an excessive amount of negative ions exists in the layer, the layer becomes negatively charged. In the silicon wafer, a certain amount of positive charge that corresponds to the negative charge is generated in the vicinity of the interface with the glass plate. Accordingly, the surface of the silicon waferand the surface of the glass plateattract each other by the Coulomb force acting between the positive charge and the negative charge and the both surfaces are brought into contact and tightly fixed.

shows the silicon waferand the glass plateafter the anodic bonding. The degree of vacuum in spaces formed by the grating of silicon between the silicon waferand the glass plateis 0.01 to 0.1 pascals as described above.

is a SEM image of the silicon waferand the glass plateafter the anodic bonding. The image ofcorresponds to.